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From:   "Alastair D'Silva" <alastair@au1.ibm.com>
To:     David Hildenbrand <david@redhat.com>,
        Michal Hocko <mhocko@kernel.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Christophe Leroy <christophe.leroy@c-s.fr>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Date:   Mon, 02 Sep 2019 09:54:34 +1000
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Organization: IBM Australia
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Subject: RE: [PATCH] powerpc: Perform a bounds check in arch_add_memory
Sender: linux-kernel-owner@vger.kernel.org
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On Tue, 2019-08-27 at 09:13 +0200, David Hildenbrand wrote:
> On 27.08.19 08:39, Alastair D'Silva wrote:
> > On Tue, 2019-08-27 at 08:28 +0200, Michal Hocko wrote:
> > > On Tue 27-08-19 15:20:46, Alastair D'Silva wrote:
> > > > From: Alastair D'Silva <alastair@d-silva.org>
> > > > 
> > > > It is possible for firmware to allocate memory ranges outside
> > > > the range of physical memory that we support
> > > > (MAX_PHYSMEM_BITS).
> > > 
> > > Doesn't that count as a FW bug? Do you have any evidence of that
> > > in
> > > the
> > > field? Just wondering...
> > > 
> > 
> > Not outside our lab, but OpenCAPI attached LPC memory is assigned
> > addresses based on the slot/NPU it is connected to. These addresses
> > prior to:
> > 4ffe713b7587 ("powerpc/mm: Increase the max addressable memory to
> > 2PB")
> > were inaccessible and resulted in bogus sections - see our
> > discussion
> > on 'mm: Trigger bug on if a section is not found in __section_nr'.
> > Doing this check here was your suggestion :)
> > 
> > It's entirely possible that a similar problem will occur in the
> > future,
> > and it's cheap to guard against, which is why I've added this.
> > 
> 
> If you keep it here, I guess this should be wrapped by a
> WARN_ON_ONCE().
> 
> If we move it to common code (e.g., __add_pages() or add_memory()),
> then
> probably not. I can see that s390x allows to configure
> MAX_PHYSMEM_BITS,
> so the check could actually make sense.
> 

I couldn't see a nice platform indepedent way to determine the
allowable address range, but if there is, then I'll move this to the
generic code instead.

-- 
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australia
mob: 0423 762 819